In a typical conventional closed loop air/fuel ratio control system for an internal combustion engine, the air/fuel ratio of the mixture is etermined by correcting a basic or standard amount of fuel to be supplied to the engine cylinders in accordance with various information relating to engine parameters and the concentration of a given gas in the exhaust gases. In some conventional closed loop air/fuel ratio control systems, the above mentioned information or data are stored in a storage device for different operating conditions, and then the amount of fuel to be supplied to the engine cylinders is determined from the appropriate data read out from the storage device, such as RAM. Although these data stored in the storage device are refreshed each time the engine operates in a given operational condition, some of the data stored are not refreshed if the engine does not operate in the corresponding operational conditions.
For instance, when a motor vehicle is driven at a high altitude, more air is needed with respect to the amount of fuel in order to maintain a desired air/fuel ratio, such as the stoichiometric value because of the low air density. Therefore, data, which may be referred to as correction factors, are renewed to compensate for such deviation of the air fuel ratio. However, the engine may not be operated at all speeds or amounts of air intake. As a result, the data corresponding to engine conditions at which the engine has not been operated at a high altitude, have not yet been renewed, and thus continue to represent data for a low altitude. Therefore, when the engine speed or intake air amount changes to a new value which has not been experienced at a high altitude, feedback control of the air/fuel ratio cannot be performed in a suitable manner during transient periods due to time lag associated with integral processing of the gas sensor output to update the data. That is, the feedback control in the above-mentioned conventional system cannot catch up with the actual variation in air/fuel ratio.